Coplanar log periodic dipole array



Sept. 20, 1966 A. RAUCH 3,274,605

COPLANAR LOG PERIODIC DIPOLE ARRAY Filed June 7, 1963 INVENTOR,

LEXANDE RAUCH ATTORNEY) United States Patent O 3,274,605 COPLANAR LOG PERKOJDIC DIPOLE ARRAY Alexander Rauch, Red Bank, N.J., assignor to the United States f America as represented by the Secretary 0f the Army Filed Inne 7, 1963, Sex. N0. 286,431 1 Claim. (C1. 343-7925) The invention descr-ibed herein may be manufactured and used by or for the Governrnent for g0vernmental purposes without the payment of any royalty thereon.

This invention relates to antennas of a type that can be described as logarithmically periodic since their antenna arrays employ logarithmically periodic antenna elements. More particularly, this invention relates to an improvement in a coplanar dipole array disclosed by D. E. Isbell in Institute of Radio Engineers Transaction, Antennas and Propagation, AP-8, N0. 3, pp. 260-267, May 1960, called Log Periodic Dipole Arrays.

The log periodic antenna of Isbell comprises a coplanar array of side-by-side dipole radiating elements with lengths and spacings related by a constant scale factor and repetitive in a logarithmic manner, that is, the physical dimensions and spacings of the dipole elements are uniformly decreased in accordance With a predetermined logarithmic ratio. These elements are mounted on a pair of parallel linear conductors spaced symmetrically about an axis, and fed at one end by a transmission line. Although Isbells array is compact and simple, its use is limited due to the fact that it presents a snbstantially contant impedance match only over a narrow band Width. For exarnple, in the above mentioned Isbell article, the antenna is only effective'ly matched over a band width having a ratio of approximately 1.6 to 1. The pfesent invention provides an improved log, periodic dipole array which may be operated over a greater band wid-th and still provide a substantially constant impedance.

In accordance with this invention, the two linear comdnctors which feed and support the dipole radiating elements of the log periodic array diverge symmetrically tfrom the axis of the antenna, While still rnaintaining the elements on respective conductors in a coplanar relationship. The structure, then, includes two diverging feed conductors such that the spacing between respective distal ends of the conductors and the antenna axis is greater than the spacing between the respective conductors and the axis at the feed ends of the conduotors. The cousequence of this uniform divergence of the conductors relative to the axis is to provide a substantially constant impedance over a wide frequency range.

The prinoipal object of this invention -is the provision 0f an antenna of the foregoing type which is conveniently matched to the input feed line over a broad range of frequencies.

For a more detailed descr-iption of the invention together with other and further objects thereof, reference is had to the single tt1g-ure of the accornpanying drawing, which is a side view of the antenna in accordance With this invention with a two wire feed system illustrated schematically.

Referring now to the drawing, there is shown a log periodic antenna having a pair of open-ended linear conductors 12 and 14 which are symmetrically arranged about the antenna axis 16. Projecting from conductors 12 and 14 over the length of the antenna 10 are a series of logarithmically spaced dipole radiating elements 18, which are aligned in a coplanar array on the conduotors. Conductors 12 and 14 are symmetrically positioned with 3Z74,605 Patented Sept. 20, 1966 respect to axis 16 to form equal acute angles with the axis, and the conductors converge toward each other from the left end of the antenna 10, as viewed. An excitation source 22 is coupled directly by means of a two Wire transrnission line 20 to the coverging end 0f the antenna 10. It will be understood, of course, that antenna 10 may be used for receiving purposes as Well as transmitting purposes, the latter being described simply for convenience as illustrative of one application of the antenna. Any suitable means, such as dielectric -supports (not shown), may be used .to rnaintain the conductors 12 and 14 in the prescribecl angular position relative to axis 16. The angle 24) of convergence of the conductors 12 and 14 is selected such that the impedance of the antenna 10 is matched over a preselected wide range of frequencies to the input transrnission line 20.

By way of exarnple, one antenna model that was built and successfully operated over a frequency range of 50 to 1100 megacycles per second employed an angle of 2q5 equal to 1, between the feed conduotors 12 and 14. This antenna utilized a coax-ial cable transmission line and was found to have a substantially constant impedance over a band Width ratio of 22 to 1. The antenna impedance of this model varied from 165 ohms at the lower frequency end to ohms at the high frequency end, thereby improving the impedance match of Isbells log period-ic dipole array over a wide range of operating frequencies.

While there has been described what is at present cousidered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various ehanges and modifications may |be made therein without departing from the invention, and it is therefore aimed in the appended clairn to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is c'laimed is:

A wideband constant impedance logarithmically periodic antenna comprising a pair of open ended linear coplanar conductors spaced symmetrically about an axis, each 0f said conductors having its open end spaced further from said axis than -its feed end so as t o from an appreciable acuate angle having an upper limit of 5 between each said conductor and said axis, a plurality of straight open ended radiating elements logarithmically spaced along said conductors at right angles .to the plane of said conductors and having successive ones of said elements extending in alternate directions from opposite sides of said plane, the length of said radiating elements decreasing logarithmically from said open end to said feed end, said conductors and radiating elements being arranged in two-fold symmetry about said axis and transmission line means connected directly across said feed ends of said antenna.

References Cited by the Examiner UNITED STATES PATENTS 3/ 1 961 Du Hamel et a1. 343-7925 2/1963 Du Hamel 34379 2.5

OTHER REFERENCES Isbell: IRE Transactions on Antennas and Propagation, Log Per-iodic Dipole Arrays, vol. AP8 N0. 3, May 1960, pp. 260467.

HERMAN KARL SAAIJBACH, Prima1ry Examzner. E. LI'EBERMAN, L. ALLAHUT, Assistant Examiners. 

